1. Field of the Invention
This application relates in general to a heat exchanger and, more particularly, to a header for mechanically joining with a series of tubes in the formation of an air-to-air after-cooler or an exhaust gas recirculation cooler, wherein leakage at the tube/header joint is reduced.
2. Description of Related Art
Current methods for securing a header to a series of tubes in high temperature air-to-air after-cooler (ATAAC) applications include mechanically expanding or rolling the tube ends into apertures formed within a header. Specifically, a tube, being composed of a brass or copper is mechanically expanded or rolled into a header being composed of a plain carbon steel. U.S. Pat. No. 3,857,151 to Young et al. shows a process for joining a series of tubes to a header wherein tubes, which are flat or oval, have end portions which are transformed into a generally round shape such that they can be inserted into apertures within a header. The tube ends are then expanded by inserting an expanding tool into the end of the tube to mechanically expand this tube end into contact with the aperture to mechanically join the tubes to the header.
Due to the differing metals used for the tubes (i.e., brass or copper alloys) and the headers (i.e., carbon steel), these materials have different thermal expansion coefficients. When exposed to high temperatures, during use of the heat exchanger, the different thermal expansion coefficients of the two materials cause a significant amount of stress to be applied to the tube material. Specifically, the brass or copper alloy tube has a tendency to grow diametrically about 1.75 times what the carbon steel header hole wants to expand. This difference in thermal growth causes the brass or copper alloy to be placed under compression by such an amount that it can yield (i.e., remain in a compressed state after release of pressure thereto) so that when the two metals return to ambient temperature, the tube will shrink to a smaller diameter than before and a leak path develops at the tube/header interface. The mechanically rolled joint now leaks.
Current solutions to the problem are to allow the leak to occur until such a point where the unit would need to be reworked or replaced due to the leak becoming too large. This problem starts small, but will increase with time and the number of cycles that the ATAAC goes through. Also, since inlet temperatures of ATAAC's are generally increasing due to new engine Tiers (engine classification on levels based on emission standards), the problem of leakage due to differing thermal coefficients of expansion will continue to increase. Another type of heat exchanger used in high temperature applications is an exhaust gas recirculation cooler (EGR). The leakage problems discussed above would also be present in EGR cooler systems.
One solution for plugging these leaks is the application of a bonding agent or metal filler material, such as a brazing alloy, to fill in the extra gap. A preferred technique for attaching tubes to a header is a technique known as a CUPROBRAZE™ technique. CUPROBRAZE™ is a manufacturing process that is used to braze copper and brass at temperatures that are generally lower than normal brazing operations, but do not exceed the softening temperatures of the components being joined. This process involves depositing a braze paste on the tubes, which are then assembled and heated to a suitable brazing temperature. The tubes used in the CUPROBRAZE™ process are based on the copper zinc iron (CuZnFe) system; particularly an alloy containing 14-31% by weight zinc, 0.7-1.5% by weight iron, 0.001-0.050% by weight phosphorous and 0-0.09% by weight arsenic, the balance being copper and incidental impurities. The paste used as the brazing compound is known as OKC 600, as discussed in U.S. Pat. No. 5,378,294 to Rissanen and U.S. Pat. Nos. 5,429,794 and 6,264,764 to Kamf et al. This compound contains binders and a metal braze alloy based on the CuSnP system, for example, about 75% copper, about 15% tin, about 5% nickel and about 5% phosphorus. Other compounds and methods are being developed for use with the CUPROBRAZE™ technique. These compounds are the subject of U.S. Pat. Nos. 7,032,808 and 6,997,371 to Shabtay and U.S. Patent Application Publication Nos. 2005/0283967 and 2006/0249559 to Panthofer. In a typical. CUPROBAZE™ process, a thin gauge brass header is used. Preferably, the header is less than approximately ¼″ thick to enable the oval holes to be punched therethrough and/or extruded so as to produce a collar. The tubes are then inserted through these collared holes and brazed into this header with the brazing paste. Other alloys currently in use to bond brass and copper to plain carbon steel generally have some kind of high level of silver content. Consequently, their use becomes price prohibitive. Also, the application of these materials would add labor costs and additional steps to the manufacturing process. Furthermore, these filler materials can also crack in high stress applications causing leaks to occur that are larger than those seen in the mechanical bonded joints.
For these reasons, it is desirable to mechanically join the tube to the header joint without the use of any other bonding or filling agent, such as brazing alloy, solder, adhesive and the like. Accordingly, there is a need in the art for a mechanical joining process which reduces or eliminates the aforementioned gaps between the tube-to-header joint caused during high heat exposure of the ATAAC or EGR exchanger during use.